1. Field of the Invention
The present invention relates to a lens drive unit, a lens barrel, and an image forming device, and more particularly relates to a lens drive unit capable of storing lens groups when not in use and of extending the lens groups by a larger amount when positioning the lens groups for use, a lens barrel that uses this lens drive unit, and an image forming device such as a camera that uses this lens barrel.
2. Description of the Related Art
A lens barrel that is incorporated into an image forming device such as a digital camera includes an imaging device, multiple lenses for forming an image on the imaging device, and a lens drive unit for moving the lenses to a predetermined position. Many of recent lens barrels used for compact cameras are each constituted by two rotary cylinders that rotate and move forward and backward in an optical axis direction, one or two linearly-moving guide barrels that move forward and backward in the optical axis direction without rotating, and one or more lens holding barrels that hold lenses and move forward and backward in the optical axis direction without rotating.
Lens barrels have been widely used which are stored compactly when not in use, and which, when in use, extend in the optical axis direction and hold each lens in a predetermined position to set a whole lens system at a predetermined focal distance. The miniaturization of lens barrels has been accelerated with the miniaturization of cameras. A multistage collapsible lens barrel with two or more rotary cylinders that meets the demand for miniaturization has been put into practical use. For example, a lens barrel described in Japanese Patent Application Publication No. 2006-039153 includes a front lens group and a first lens frame that holds this lens group. The first lens frame is movable forward and backward by being held by two rotary cylinders, two linearly-moving guide barrels, and one fixed cylinder.
A multistage collapsible lens barrel makes it possible to achieve miniaturization and to secure the moving distance of a lens. Thus, such a lens barrel is extremely suitable for a compact camera. Meanwhile, the demand for lenses with higher magnifications has been increased, and this increases the necessity of securing a larger lens extension amount. Although achieving the required lens extension amount, the multistage collapsible lens barrel may increase the area of a lens barrel, and thus may result in an increase in size of a camera. To avoid this, a multistage collapsible lens barrel needs to be designed to secure a larger extension amount while having a minimum possible size. In the multistage collapsible lens barrel, lenses can extend to the maximum extent toward an object when a rotary cylinder for moving the lenses to a desired position engages with or is fitted with other members that operate in association with this rotary cylinder by a minimum necessary amount. Even in this maximum extended state, a lens holding frame needs to be fitted with not only the rotary cylinder but also a linearly-moving guide barrel for moving the lenses straight. Since the linearly-moving guide barrel has functions of guiding the lens holding frame to move straight and of preventing inclination of the lens holding frame by being fitted with the lens holding frames, the linearly-moving guide barrel needs to engage with or be fitted with the lens holding frame by a sufficient amount. Thus, there has been a problem that the necessary fitting amount of the lens holding frame with the linearly-moving guide barrel reduces the actual movement amount of the lenses from the maximum lens extension amount actually achievable by the rotary cylinder, whereby the lenses can be moved only by a smaller lens extension amount.
In order to explain more detail, FIGS. 4 and 5 show a configuration of a lens barrel having the same configuration of a lens barrel disclosed in Japanese Patent Application Publication No. 2006-039153.
In FIGS. 4 and 5, a first lens group 1 is held by a first lens holding frame 5, this lens holding frame 5 is held by a rotary cylinder 6 and a first linearly-moving guide cylinder 7, and the rotary cylinder 6 fits into a helicoid formed in an internal diameter part of a linearly-moving guide member 9. The first linearly-moving guide cylinder 7 is held so as to allow rotation of the rotary cylinder 6 and to be guided by the linearly-moving guide member 9 to move straight in an optical axis direction while integrally holding the rotary cylinder 6. A rotary member 8 fits into a helicoid formed in an internal diameter part of a fixed cylinder 10. The linearly-moving guide member 9 is held so as to be rotatable relative to the rotary member 8 and to be guided by the fixed cylinder 10 to move straight integrally with the rotary member 8 in the optical axis direction. A rotary key groove 8a is provided in a surface perpendicular to the optical axis direction in the internal diameter part of the rotary member 8. A rotary key 9b provided in the outer circumference of the linearly-moving guide member 9 fits into the rotary key groove 8a. In this way, the rotary member 8 and the linearly-moving guide member 9 can integrally move in the optical axis direction while rotating relative to each other.
The rotary member 8 and the rotary cylinder 6 are connected by a rotation driving pin transmitting a rotational force of the rotary member 8, and can move forward and backward in the optical axis direction when the rotary member 8 rotates because of the helicoid connection. Furthermore, the first lens holding frame 5 has a lens driving pin, and this lens driving pin engages with a cam groove provided in an internal diameter part of the rotary cylinder 6 and engages with a guide groove of the first linearly-moving guide cylinder 7 provided in the optical axis direction. Thereby, a lens can move to a position corresponding to the shape of the cam groove.
The upper half of FIG. 4 shows a telephoto state where the focal distance of a lens becomes the maximum, and the lower half thereof shows a stored state where the lens barrel is retracted to have the minimum lens interval when no photograph is taken. FIG. 5 shows a wide-angle state where the focal distance becomes the minimum. In FIGS. 4 and 5, a rotary cam cylinder 11 is fitted with the inside of the first linearly-moving guide cylinder 7. The rotary cam cylinder 11 is held to be movable integrally with the first linearly-moving guide cylinder 7 in the optical axis direction, and is connected to the rotary cylinder 6 and thus is rotatable integrally with the rotary cylinder 6. The rotary cam cylinder 11 includes a cam groove, through which a second cam follower 121 provided in a second lens holding frame 12 for holding a second lens group 2 passes. This cam follower 121 passes through the cam groove of the rotary cam cylinder 11, and engages with a linearly-moving groove 71 in the first linearly-moving guide cylinder 7. With rotation of the rotary cam cylinder 11, the intersecting position of the cam groove of the rotary cam cylinder 11 and the linearly-moving groove 71 of the first linearly-moving guide cylinder 7 moves in the optical axis direction. Thereby, the second cam follower 121 fitted into this intersecting position and the second lens holding frame 12 integral therewith move straight in the optical axis direction.
In a mode shown in FIG. 4, the first lens group 1 moves to the maximum extended position toward the object with rotation of the rotary member 8, and extends an interval between the first lens group 1 and the second lens group 2 to achieve a long focal distance. In this state, the fitting amount of the first lens holding frame 5 with the first linearly-moving guide cylinder 7 is extremely small, which causes a problem that the first lens holding frame 5 is likely to incline. Furthermore, the first linearly-moving guide cylinder 7 may be short with a configuration in which a barrier mechanism that protects a lens when no photograph is taken is provided forward of the first lens group 1 and is located in the same position as that of other rotary cylinders in the stored state. Accordingly, there is a problem that the first lens holding frame 5 cannot move forward of a front end of the rotary cylinder 6 since otherwise the first lens holding frame 5 separates from the first linearly-moving guide cylinder 7.